1. Field of the Invention
The present invention relates to an exhaust gas cleaning catalyst component which decomposes NOx contained in an exhaust gas in the presence of a reducing agent such a hydrocarbon (HC) in an atmosphere comprising excess oxygen, a process for the production of the same and a process for the production of an exhaust gas cleaning catalyst member, especially a honeycomb member comprising such a catalyst component.
2. Description of the Related Art
A three way catalyst is known as an exhaust gas cleaning catalyst for an internal combustion engine of, for example, an automobile. The three way catalyst is able to clean a hydrocarbon(HC), carbon monoxide(CO) and NOx contained in the exhaust gas at around a stoichiometric air-fuel ratio. However, it is not suitable for cleaning of NOx of the exhaust gas from the engine operated at a high oxygen concentration, so called a lean burn engine.
To the contrary, a zeolite supporting Pt and Rh is known as a catalyst which cleans NOx of the exhaust gas from a lean burn mode under an exceed oxygen concentration (see Japanese Patent Kokai Publication No. 243545/1992).
The inventors have made intensive studies as to a catalyst comprising the metal containing silicate such as a zeolite described above which carries a noble metal active component (such as Pt), and already found that combination of Pt and Ir, and optionally further combined with Rh, improves activity and heat resistance of the catalyst.
However, the metal containing catalyst carrying the noble metal active component as described above shows a low temperature (for example below 250.degree. C.) at which a NOx cleaning ratio is maximum (so called a peak temperature). Thus, when the catalyst is heated to a high temperature as in the case of an exhaust gas cleaning catalyst for an automobile, a predetermined NOx cleaning cannot be achieved.